1. Field of the Invention
The present invention relates to a cathode-ray tube applied to a three-tube type projector and so on and a blue phosphor for use in a phosphor surface thereof.
2. Background of the Invention
There has been known a three-tube type video projector in which single-color cathode-ray tubes for red (R), green (G) and blue (B) are arranged in line and projection lenses provided in front of the respective single-color cathode-ray tubes enlarge and form an image on a screen.
It is urgently required to improve brightness of blue color in such video projector.
ZnS:Ag, Al has been used as a blue phosphor for the projector. Various attempt described later on have been made in order to improve brightness in a single-color cathode-ray tube for blue color.
(i) Method of Improving Efficiency of Extracting Light
In this method, an interference film in which oxidized films of aluminum and titanium or of tantalum and silicon are alternately laminated in a multilayer structure is deposited between a panel inner surface of a cathode-ray tube and a phosphor surface thereof, and this interference film gives directivity to light emitted from a phosphor film by excitation. Thus, the efficiency of extracting light is improved, thereby brightness being increased.
(ii) Method of Increasing Density of Phosphor Film
In this method, density of a phosphor film is increased by making particles of phosphor smaller and increasing a package density of phosphor. Thus, a so-called browning phenomenon caused when electron beams passes through a phosphor film and then collides with a panel glass is prevented, and a current density is increased to that extent, thereby the brightness being improved.
(iii) Method of Optimizing Thickness of Phosphor Film
In this method, a thickness of the phosphor film is optimized, thereby the brightness being improved.
(iv) Method of Preventing Panel Glass from Browning
In this method, alumina sol is deposited on a panel inner surface, thereby the panel glass being prevented from browning. The current density is increased to that extent, thereby the brightness being improved.
However, the method (i) employing the interference film encounters a problem of dependency upon an angle of view and also encounters disadvantages such as change of transmission characteristics presented when deterioration of the interference film (so-called increase of film density) is caused by irradiation of electron beams, occurrence of the browning phenomenon, and the like.
The method (ii) of increasing the density of the phosphor film encounters the disadvantage that the brightness of the phosphor itself becomes unsatisfactory by making particles of the phosphor smaller and the brightness cannot be improved to an expected degree.
The method (iii) of optimizing the thickness of the phosphor film encounters the disadvantage that it is difficult to optimize the thickness thereof when the phosphor film is actually manufactured.
The method of (iv) of preventing the panel glass from browning encounters the disadvantages that in the thermal process the alumina sol film is easily cracked and the phosphor film easily becomes uneven.